1. Field of the Invention
The present invention relates to a piezoelectric sensor for sensing acceleration or vibration and, more particularly, to a piezoelectric sensor to be mounted on a substrate such as a printed circuit board.
2. Description of the Related Art
An acceleration sensor which is a kind of piezoelectric sensor is usually used by packaging it or mounting it on a mounting substrate such as a circuit board which has a circuit for processing acceleration signals generated by the sensor.
Mounting of an acceleration sensor on a mounting substrate has caused two major problems in the prior art.
The first problem is as follows. The design of an acceleration sensor to be mounted on a substrate is made such that the sensor senses acceleration acting in directions perpendicular to the plane of the mounting substrate or acceleration acting parallel to the plane of the mounting substrate, depending on factors such as the use of the sensor and specifications of the circuit board. It is therefore necessary that the acceleration sensor be mounted on the circuit board so as to be sensitive to acceleration acting in either one of the above-mentioned directions.
Meanwhile, the acceleration sensor has a rectangular parallelopiped configuration having six surfaces, and is adapted to be jointed to the circuit board at a specific one of the six surfaces or a surface opposite to this surface. This is because the acceleration sensor has a flattened rectangular parallelopiped configuration having a rectangular bottom and top surfaces and side surfaces of a height which is small as compared with the two sides of the rectangle comprising the top and bottom surfaces, so that the sensor can be soldered to the circuit board while being stably held on the circuit board. For instance, the height ranges from 0.5 to 0.7 times as large as the length of the shorter side of the rectangle defining the top and bottom surfaces.
Consequently, it has been necessary to prepare two types of acceleration sensors: an acceleration sensor of the type which is capable of sensing acceleration acting perpendicularly to the plane of the circuit board and an acceleration sensor of the type which is capable of sensing acceleration acting parallel to the plane of the circuit board. An acceleration sensor of either one of these two types is selected for use. This not only raises the costs of production of acceleration sensors but also requires costs for storing and administrating these two types of acceleration sensors, resulting in a rise in the costs of various products incorporating such acceleration sensors.
The second problem is as follows: The circuit board on which an acceleration sensor is packaged may be deflected for any reason, before or after the packaging of the acceleration sensor. Specifically, the deflection of the circuit board causes a corresponding deformation of the acceleration sensor. Such a deformation of the acceleration sensor due to an external force adversely affects the piezoelectric member which senses acceleration, thus hampering sensing of acceleration. This problem is particularly serious when the acceleration sensor is intended to sense acceleration acting perpendicularly to the plane of the mounting substrate, because in such a case the direction of acceleration to be sensed coincides with the direction of deflection of the mounting substrate. Consequently, the acceleration sensor may be influenced by the deflection of the circuit board so as to erroneously produce an acceleration signal even when there is no acceleration acting on the sensor.
Accordingly, an object of the present invention is to provide a piezoelectric sensor which can reduce adverse effects due to the deflection of the substrate and a piezoelectric sensor which can be mounted on a circuit board selectively either in a direction to sense acceleration perpendicular to the plane of the circuit board or in a direction to sense acceleration parallel to the plane of the circuit board, thereby obviating at least one of the two major problems encountered with the prior art.
To this end, according to one aspect of the present invention, there is provided a piezoelectric sensor comprising: a piezoelectric element having a specific axis of sensitivity; and a package in which the piezoelectric element is packaged; wherein the package has a rectangular parallelopiped configuration with end surfaces having a height-to-width ratio approximating 1:1, and wherein external lead electrodes are formed on at least the end surfaces.
According to a second aspect of the present invention, there is provided an acceleration sensor comprising: a bimorph element having an axis of highest sensitivity extending in a direction which substantially coincides with a line normal to the plane of a circuit board; and a case assembly for fixing and supporting both longitudinal ends of the bimorph element, the case assembly being adapted to be mounted on the circuit board at both its longitudinal ends which support both longitudinal ends of the bimorph element; wherein the bimorph element has a pair of piezoelectric ceramic plates each having a signal electrode and an intermediate electrode formed on the opposite major surfaces thereof, the piezoelectric ceramic plates being joined to each other face to face at their surfaces having the intermediate electrodes such that the intermediate electrodes are coupled to each other; each of the piezoelectric ceramic plates being sectioned in the longitudinal direction of the bimorph element into three sections including a central section and both end sections at border lines which are positioned such that when the bimorph element is deformed in response to deflection of the circuit board, the quantity of charges generated in the central section equals the sum of the quantities of the charges generated in both the end sections, the central section and both end sections of each piezoelectric ceramic plates being polarized thicknesswise of the piezoelectric ceramic plate in opposite directions, the directions of polarization of the central section and both end sections of one of the piezoelectric ceramic plates being opposite to those of the other of the piezoelectric ceramic plates.
According to a third aspect of the present invention, there is provided an electronic part having a polyhedral body on opposing end surfaces of which are formed electrodes for outputting voltages of different polarities, the electronic part comprising: a conductive film formed on one of the surfaces of said body orthogonal to the end surfaces having the electrodes at a predetermined position closer to one of the end surfaces than to the other, the conductive film having an area large enough to be simultaneously contacted by a pair of probe terminals for applying a voltage.
According to a fourth aspect of the present invention, there is provided a method of examining posture of an electronic part which has to be placed in a predetermined posture in terms of up and down, left and right and front and back directions, the method comprising: bringing a pair of probe terminals into contact with a potion of an upwardly directed surface of the electronic part disposed in an examination position in an arbitrary posture, the portion being closer to one of the end surfaces having electrodes than to the other; applying a voltage between the probe terminals; and determining, based on the presence or absence of electrical current between the probe terminals, whether the electronic part as the examination object has been placed in a correct posture.
According to a fifth aspect of the invention, there is provided an apparatus for examining posture of an electronic part and which has to be placed in a predetermined posture in terms of up and down, left and right and front and back directions, the apparatus comprising: a pair of probe terminals adapted to be brought into contact with a potion of an upwardly directed surface of the electronic part disposed in an examination position in an arbitrary posture, the portion being closer to one of the end surfaces having electrodes than to the other; detecting means for applying a voltage between the probe terminals and for detecting presence or absence of electrical current between the pair of probe terminals; and determining means for determining, based on the results of the detection by the detecting means, whether the electronic part as the examination object has been placed in a correct posture.
The piezoelectric sensor in accordance with the present invention. is designed to be stably seated on a printed circuit board regardless of the posture of mounting thereof on the printed circuit board. Therefore, a single piezoelectric sensor can provide a variety of directions of axis sensitivity, by changing the posture of the piezoelectric sensor mounted on the printed circuit board, thus widening the adaptability of the piezoelectric sensor. The present invention therefore achieves a remarkable reduction in the costs of production and management of piezoelectric sensors and, hence, the price of the same.
The acceleration sensor of the present invention offers an advantage in that, even if the bimorph element is deformed due to the influence of deflection of the circuit board on which the acceleration sensor is mounted, charges generated as a result of the deformation are canceled by each other, so that no signal charges are derived from the sensor when no acceleration is acting thereon, whereby the influence of deflection of the circuit board is eliminated. However, when acceleration acts on the acceleration sensor, charges are generated in the central section and both end sections of each piezoelectric ceramic plate, based on the relationship between the directions of polarization of these sections and the tensile and compression stresses caused by the deformation. Such charges are picked up as output signal voltage, without being canceled, thus providing a high level of output signal. According to the invention, therefore, it is possible to suppress influence caused by deflection of the circuit board, while achieving a higher degree of reliability. Moreover, a high level of sensor output can be obtained when acceleration actually acts on the sensor.
The electronic part in accordance with the present invention enables, with simple electronic processing, confirmation of the posture of the electronic part which is to be mounted on a printed circuit board or to be loaded in a tape carrier correctly in a predetermined posture, with a high degree of accuracy while avoiding increase in production cost.
The examination method and apparatus of the present invention enables the confirmation of the electronic part under the condition described above. In particular, the examination apparatus offers a remarkable reduction in the installation cost as compared with the conventional examination system which relies upon image processing techniques.